Besides the obvious cosmetic aspect, contact lenses generally offer improved visual acuity compared to spectacles. In some cases, the difference is dramatic. For example, in the case of the corneal pathology of keratoconus (a corneal condition in which there is local region of high curvature), contact lenses can often succeed in providing excellent visual acuity (perhaps 20/20 using the standard Snellen eye chart) whereas spectacles are not able to provide more than a minor improvement over the uncorrected vision. In addition to improved visual acuity, contact lenses are also indicated for other diverse purposes, such as a medicine delivery system or as a "bandage" for protection of the cornea after erosion, trauma, or surgery.
Current contact lenses have shapes formed from relatively simple geometries, mostly spherical of different radii, or conic sections, etc. Consequently, there are many limitations, including poor fit for corneas with complex shape (such as might be found in keratoconus or in post-surgical corneas), edges that are uncomfortable, limited optical correction, limited ballasting/stabilization designs (for orientation of non-rotationally symmetric lenses), etc.
The most complex designs of the anterior or posterior surface of a contact lens are based on two or three surface zones. They are usually spherical, or sometimes so-called "aspherical". Although "aspherical" literally means "not a sphere", this term is used more narrowly in the contact lens field to refer to what mathematicians call surfaces of revolution of conic sections or toric surfaces.
Most contact lenses are either lathed directly, or molded from molds that were produced from pins and inserts that were lathed or ground. The lathing and grinding technology that is commonplace in the contact lens industry produces rotationally-symmetric lenses (except for toric lenses for astigmatism which are generally produced by a rather ad-hoc "crimping" method) and the shapes are fairly simple geometrically. In order to realize more general shapes, more sophisticated fabrication techniques are necessary. Computer numerical control (CNC) machining moves a cutting tool along a path on a part according to a mathematical model. The concept is that the computer instructs the machine how to make the complex shape, and the machine is capable of making such a shape. Furthermore, such high accuracy is achieved that the usual requirements of polishing the scallops (ridges) is greatly reduced or even eliminated. Some CNC machines can produce contact lenses that are non-rotationally symmetric.
There is a widely-held precept that making custom shapes is impractical and costly--that "one size fits all" is the only economical method. The advent of CNC machining shatters some deeply-held beliefs about manufacturing. In the traditional manufacturing process, the notions of mass production and economies of scale are predicated on the assumption of producing many identical copies of a product; but these ideas date from the industrial revolution. "Any color as long as it is black" is a Model T concept. Nowadays, automobiles are manufactured efficiently despite the fact that each one that rolls out of the factory door has a unique permutation of a dizzying array of options. Mass customization can be realized by integrating computers into the manufacturing process such that each contact lens can be automatically produced to custom specifications; the computer simply uses the particular set of values of the parameters of the mathematical model for each unit. Concerns about minimizing the number of different stock keeping units (SKU's) could be a thing of the past by embracing concepts of just in time manufacturing.
Although CNC machines enable the fabrication of complex surfaces, and the variation from one unit to the next, they require that a powerful mathematical model be used. In the evolution from traditional manual machining to automated CNC technology, all details must be specified competely and precisely.